In a wireless mobile communication system, in order to ensure continuous communications of a mobile terminal, the mobile terminal shall be handed over under a connected state. In order to correctly trigger the handover, the mobile terminal shall perform a Radio Resource Management (RRM) measurement. If results of the measurement always meet a certain condition (i.e., a triggering condition, also referred to as a triggering event) within a specified period (Time to Trigger, TTT), the mobile terminal reports the results of the measurement to a base station, and the base station can decide whether to start a handover process according to the results of the measurement.
FIG. 1 illustrates a schematic diagram in which a mobile terminal reports the TTT to a base station based on an RRM measurement. As illustrated in FIG. 1, if the triggering condition/event is defined as “the signal quality of the current serving cell is lower than a certain threshold within the TTT”, the mobile terminal reports the results of the measurement to the base station only when values of the measurement are always lower than the threshold in the TTT.
In this process, the TTT and the triggering condition/event can both influence success rate of the handover. For example, in a case where the triggering condition/event is “the signal quality of the current serving cell is lower than a certain threshold”, the serving cell requires extra time from obtaining a measurement report to triggering and performing the handover, thus when the threshold is too low, there may be a situation that although the mobile terminal has reported the result of the measurement, the signal quality of the current serving cell quickly deteriorates, the subsequent actions such as triggering the handover cannot be performed, and a call drop will occur at the mobile terminal. When the threshold is too high, there may be a situation that the signal quality of the current serving cell still can maintain the communication with the mobile terminal, while the signal quality of the destination cell is not good enough, and a call drop will also occur at the mobile terminal. Meanwhile, the TTT can influence effect of the handover. When the TTT is too long, an extra deterioration of the signal quality of the serving cell may appear, the signal quality of the serving cell is not enough to support a subsequent handover operation, and a call drop will occur. When the TTT is too short, the signal quality of the serving cell may fluctuate momentarily, and the signal quality can be recovered quickly. In that case, the mobile terminal is handed over from the current serving cell, while the signal quality of the destination cell is not good enough, thus a call drop will easily occur at the mobile terminal.
The setting of the TTT is related to multiple conditions such as a channel static change situation and speed of a mobile terminal. The channel static change situation is the quality change situation of the channel between the mobile terminal and the base station when the mobile terminal is static: if the channel quality fluctuates in a large range, generally the TTT shall be set to be relatively long to smooth a large channel fluctuation; whereas if the channel quality fluctuates in a small range, the TTT may be set to be relatively short. When speed of the mobile terminal is considered, the TTT shall be short if the speed of the mobile terminal is high, so as to prevent the channel quality from quickly varying with the user position, whereas if the speed of the mobile terminal is low, the TTT shall be long.
Since it is difficult to judge the channel static change situation, in an existing network, the TTT is set by the operator according to a general situation, and the TTT is corrected by the mobile terminal according to its own mobile state: the mobile state is substantially judged by counting the number of handovers within a specified time (the speed increases with the number). From the mobile state, corresponding correcting factor can be searched and then multiplied by the original TTT. The correcting factor is generally less than 1, and it decreases when the speed rises. The process for the mobile terminal to correct the TTT is as follows:
Step 1: the base station triggers the mobile terminal to detect the mobile state. The base station transmits parameters required for the TTT to the mobile terminal: a handover count period, an additional handover count period, an upper threshold, an intermediate threshold, a high-speed correction value and an intermediate-speed correction value.
Step 2: the mobile terminal counts the number of handovers within the handover count period. When the handover count period expires, if result of the counting is larger than the upper threshold, the mobile terminal judges that it enters a high-speed mobile state; if result of the counting is between the upper threshold and the intermediate threshold, the mobile terminal judges that it enters an intermediate-speed mobile state; and if neither the high-speed mobile state nor the intermediate-speed mobile state is detected when the additional handover count period expires, the mobile terminal judges that it enters a normal mobile state. Meanwhile, if the mobile terminal is subsequently handed over back to a cell after being handed over out of the cell (i.e., a ping-pong effect), the continuous handover between the two identical cells is not counted.
Step 3: if the mobile terminal is in the high-speed mobile state, it corrects the TTT using the high-speed correction value; if the mobile terminal is in the intermediate-speed mobile state, it corrects the TTT using the intermediate-speed correction value; and if the mobile terminal is in the normal mobile state, it does not need to correct the TTT.
The above mechanism has an obvious problem: the mobile terminal does not consider a cell size when counting the number of handovers. FIG. 2 illustrates a schematic diagram of an influence of a cell size on the counted number of handovers. As illustrated in FIG. 2, the mobile terminal moves through paths 1 and 2 in the same speed respectively. When the mobile terminal moves through path 1, since a radius of the cell passed through is larger, the number of handovers is obviously smaller than that when the mobile terminal moves through path 2. Thus, different mobile states may be obtained when the mobile terminal moves in two paths.
In order to overcome the above problem, multiple mechanisms are optional at present.
Mechanism 1: the base station notifies its size to all mobile terminals in a cell where the base station is located. Thus the mobile terminal can acquire the cell size of the handover, and correct the number of handovers to obtain a more accurate mobile state. This method is a direct improvement to the prior mechanism and it still depends on the handover counting by the mobile terminal.
Mechanism 2: the base station corrects parameters (e.g., the upper threshold) related to the counting according to environment of the mobile terminal. For example, when the mobile terminal moves from an area of a large cell radius to an area of a small cell radius, the base station can configure a large upper threshold for the mobile terminal. Thus the mobile terminal considers the cell size when judging the mobile state. Obviously, the method still depends on the handover counting by the mobile terminal.
Mechanism 3: an actual speed value is measured through a speed detection algorithm rather than the handover counting. For example, the mobile terminal having a speed detection capability can acquire its actual speed through a device such as GPS. The mechanism depends on the capability of the mobile terminal, but not any mobile terminal has such the capability in the actual system.
Therefore, the measurement of the mobile state of the mobile terminal (i.e., the moving speed) in the relevant art is mainly performed by the mobile terminal itself, or it requires the mobile terminal itself to have the speed detection capability, or it requires a complex counting operation. As a result, the complexity of the mobile terminal is increased.
To be noted, the above introduction to the technical background is just made for the convenience of clearly and completely describing the technical solutions of the present invention, and to facilitate the understanding by a person skilled in the art. It shall not be deemed that the above technical solution is known to a person skilled in the art just because it has been illustrated in the Background section of the present invention.    [Non-patent literature 1]: 3GPP TS 36.331 V10.1.0 (2011-3), Radio Resource Control (RRC) specification. (Release 10);    [Non-patent literature 2]: R2-113181, Discussion on mobility estimation for HTN, Alcatel-Lucent, Alcatel-Lucent Shanghai Bell;    [Non-patent literature 3]: 3GPP TS 36.304 V10.2.0 (2011-06), User Equipment (UE) procedure in idle mode (Release 10).